The present invention relates to fabrication methods for high temperature superconducting materials, more particularly to melt-texturing fabrication methods for bulk high temperature superconducting materials.
According to conventional methods of preparing ceramic superconductors, suitable powders of oxides are prepared; these powders must be spatially and chemically homogeneous. See Mitchell, T. E., D. R. Clarke, J. D. Embury, and A. R. Cooper, "Processing Ceramic Superconductors," JOM 41 (1) 6-10 (1989), incorporated herein by reference. Such methods of powder production have included vapor or sol-gel routes of coprecipitation or mechanical blending.
In the case of YBa.sub.2 Cu.sub.3 O.sub.7 the latter procedure has been more frequently employed. Typically BaCO.sub.3, Y.sub.2 O.sub.3, and CuO are mixed together and calcined at 800.degree. C. for six hours in air. Calcining is followed by ball milling with zirconia in dry acetone for one hour, die pressing the mixed powder, then sintering at 950.degree. C. These procedures are followed by oxygenation at 450.degree. C., remilling, and repeating the sintering-oxygenating step. Production of sub-micron size powder is highly desirable. The powder is once again die compressed up to 70% dense material. Pressure sintering has also been tried. This procedure involves sinter-forging, hot isostatic pressing and hot extension. Control of oxygen content is believed to be very important in elevating T.sub.c.
Melt-quenching is another method by which superconducting ceramics can be prepared. This involves melt-quenching rather than slow cooling after annealing. See Komatsu, T., T. Ohki, K. Imai, and K. Matusita, "Preparation of Superconducting Ba-Gd-Cu-O and Ba-Yb-Cu-O Ceramics by the Melt Quenching method," J. Mater. Sci. Lett. 8 (1) 1-3 (1989), incorporated herein by reference. Here the nominal stoichiometric composition of oxides, e.g., Ba.sub.2 YCu.sub.3 O.sub.7-x, is first quenched and then annealed at approximately 900.degree. C. Ba-Gd-Cu-O samples annealed 72 hours superconduct to 94.degree. K. with current densities of 50 A-cm.sup.-2. Ba-Yb-Cu-O samples annealed 48 hours superconduct to 90.degree. K.
Elevation of T.sub.c has also been accomplished by thermal cycling of YBa.sub.2 Cu.sub.3 O.sub.7-y below 240.degree. K. See Bhargava, R. N., S. P. Herko, and W. N. Osborne, "Improved High-T.sub.c Superconductors," Phys. Rev. Lett. 59 1468 (1987), incorporated herein by reference. It is believed that compositional inhomogeneities are responsible for T.sub.c lowering. Therefore, a semi-wet method has been tested as a way to reduce these inhomogeneities more effectively than does repeated grinding and firing. See Pandey, D., V. S. Tiwari, and A. K. Singh, "A Semi-wet Route to the Synthesis of YBa.sub.2 Cu.sub.3 O.sub.7-y Ceramics," J. Phys. D: Appl. Phys. 22 182-186 (1989), incorporated herein by reference. For the semi-wet method, yttrium and barium chloride salts are mixed in the correct stoichiometric ratio and then are coprecipitated with ammonium carbonate solution. X-ray diffraction of the solid mixture shows that the compositional inhomogeneities are greatly reduced with this technique.
Other methods have been investigated for improving superconductivity properties. These include those methods disclosed by the following references, all of which are incorporated herein by reference: Murugaraj, P., J. Maier, and A. Rabenau, "Preparation and Characterization of Highly Oriented Ceramics and Large Crystals of YBa.sub.2 Cu.sub.3 O.sub.x Superconductor," Duro-Ceramics 2 421-5 (1989) (production of highly ordered materials with large crystals); Van der Biest, O. O., J. Fransaer, T. Eggermont, and O. Arkens, "A Versatile Sol-Gel Method for Synthesis of Ceramic Superconductors," Euro-Ceramics 2 405-9 (1989) (sol-gel preparation utilizing EDTA); Merzhanov, A. G., "Self-Propagating High-Temperature Synthesis of Ceramic (Oxide) Superconductors," Ceram. Trans., 519-49 (1990) (self-propagating high-temperature synthesis with the use of Cu as a fuel and Y and Ba as oxidizers); Pak, S. S., and K. S. Mazdiyasni, "Synthesis and Characterization of Sol-Gel Derived, Submicron Size, High T.sub.c Ceramic Superconductor Powders," Ceram. Trans., 757-64 (1990) (use of powders of high chemical and phase purity synthesized by homogeneous hydrolytic decomposition of the component oxides); Mihalich, "Process for Preparing Crystalline Ceramic Superconductor Materials by Fluidized-Bed Calcination," U.S. Pat. No. 4,931,426 (1990) (use of a fluidized-bed calcination process); Kayima, P. M., and S. Qutubuddin, "Preparation of Monosized, Spherical, Colloidal Particles of Yttrium Barium Cuprate Superconducting Oxide Ceramic Precursors," J. Mater. Sci. Lett. 8 (2) 171-2 (1989) (homogeneous solution precipitation of metal ions by in situ decomposition of urea at elevated temperatures); Kodas, T. T., E. M. Engler, V. Y. Lee, R. Jacowitz, T. H. Baum, K. Roche, S. S. P. Parkin, W. S. Young, S. Hughes, J. Kleder, and W. Auser, "Aerosol Flow Reactor Production of Fine Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 Powder: Fabrication of Superconducting Ceramics," Appl. Phys. Lett. 52 (19) 1622-4 (1988) (use of an aerosol flow reactor operating at 900.degree.-1,000.degree. C. which gives a uniform chemical composition and a submicron-to-micron average particle size formed from the thermal decomposition of aerosol droplets containing nitrate salts of the required metals); Falter, L. M., D. A. Payne, T. A. Friedmann, W. H. Wright, and D. M. Ginsberg, "Preparation of Ceramic Superconductors by the Pechini Method," Br. Ceram. Proc. 261-9 (1989) (employment of a liquid-solution resin-forming process from which powders are formed into dense ceramics by hot-pressing); Jin, S., T. H. Tiefel, R. C. Sherwood, R. B. van Dover, M. E. Davis, G. W. Kammlott, and R. A. Fastnacht, "Melt-textured Growth of Polycrystalline YBa.sub.2 Cu.sub.3 O.sub.7-x with High Transport J.sub.c at 77 K., " Physical Review B 37 7850-7853 (1988) (melt-texturing); and Richardson, T. J. , and L. C. De Jonghe, "Traveling Reaction Zone Method for Preparation of Textured Ceramic Superconductor Thick Films," J. Am. Ceram. Soc. 73 (11) 3511-13 (1990), and Richardson, T. J. , and L. C. De Jonghe, "Travelling Solvent Zone Texturing of Ceramic Superconductor Thick Films," J. Mater. Sci. Lett. 84 (10) 369-370 (1991) (melt-texturing using a steep temperature gradient furnace).
Other references of note, incorporated herein by reference, are: Zhang, J. M., B. W. Wessels, D. S. Richeson, and T. J. Marks, "Preparation and Properties of Superconducting Bi-Sr-Ca-Cu-O Thin Films on Polycrystalline Silver Substrates by Organometallic Chemical Vapor Deposition," Journal of Crystal Growth 107 705-709 (1991); Nakagawa, S, T. Takeuchi and M. Name, "Preparation of YBaCuO Thin Films with Excellent Crystallinity on Amorphous Substrates Prepared by Facing Targets Sputtering," IEEE Transactions on Magnetics 26 (5) 1430-1432 (September 1990); Brown et al., "Process for Preparing Metal-Ceramic Coatings Electrically Superconducting above 77 Degrees Kappa," U.S. Pat. No. 4,981,840; Urano et al., "Method of the Production of Ceramic Superconductor Filaments," U.S. Pat. No. 4,968,662; Hoenig, "Method for Producing a Layer-Like Composition of Oxide-Ceramic Superconducting Material," U.S. Pat. No. 4,916,114; Akinc et al., "Method for Preparing Superconductors Ceramic Composition," U.S. Pat. No. 4,906,608; DiCarolis, "Method of Producing Superconducting Ceramics by Co-Preciptation of Carbonates," U.S. Pat. No. 4,904,638; Snyder et al., "Preparation of Precursors for Yttrium-Containing Ceramic Superconductors," U.S. Pat. No. 4,900,536; Pederson et al., "Preparation of Thin Ceramic Films Via an Aqueous Solution Route," U.S. Pat. No. 4,880,772; Heavens et al., "Preparing Superconducting Ceramic Materials," U.S. Pat. No. 4,810,339; Walker, "Apparatus for Exposing Liquids to Direct Contact with Air or Gases," U.S. Pat. No. 2,206,440.
Jin et al. report that melt-textured growth of YBa.sub.2 Cu.sub.3 O.sub.7-x from a supercooled melt created an essentially 100% dense structure of long, locally aligned, needle-shaped grains. Jin et al. describe a method wherein precursor samples of material with formula YBa.sub.2 Cu.sub.3 O.sub.7-x were prepared in rectangular bar or sheet shape, lead wires were attached to the samples by In solder, current was applied, the samples were melt-processed by heating either to the single-phase region or two-phase region, the samples were held at the selected temperatures (1320.degree. C., 1180.degree. C., 1110.degree. C., 1030.degree. C.) for 0.5-120 minutes in flowing oxygen environment, the samples were cooled in a temperature gradient of about 50.degree. C./cm to room temperature, and the samples were subsequently given additional heat treatment for purposes of homogenization/stress relief/enhanced oxygen content. The process of cooling the samples in a temperature gradient is termed "melt-textured growth" in accordance with the manner in which the crystals are made to grow and align during solidification of the melt.
Mitchell et al., citing Jin et al., note that texture development in superconducting materials involves the concept of flux flow wherein a transport current provides a driving force acting on the flux lines. "Weak links" in terms of various chemical or structural inhomogeneities--e.g., grain boundaries, dislocations, particles--exert pinning forces which act to prevent flux flow. Utilization of single crystals avoids the weak link problem. Practical approaches to fabricating textured materials in the form of "pseudo-single" crystals, besides melt-texturing, have included deformation-texturing, magnetic-field alignment and epitaxial growth of films on substrates.
Jin et al. note that the exact mechanism responsible for the suppression of weak link behavior cannot be exactly pinpointed, but melt-texturing yields at least three beneficial structural changes: (1) formation of dense structure with enhanced connectivity between superconducting grains; (2) orientation of crystals along the preferred superconducting direction; and, (3) formation of new, cleaner grain-boundary area as grain length increases and total grain-boundary area decreases and as decomposition of impurities is facilitated by exposure to higher temperatures.
Of particular interest herein is the process disclosed by Richardson et al., a "melt-texturing" directional solidification process which increases the temperature at which superconductive properties can be maintained in metallic materials. According to Richardson et al., rare earth metal oxides are placed into a liquid solvent. The solution is then placed into an unevenly heated oven in which a strong temperature gradient can be formed. The oven is then rapidly heated to approximately 800.degree. C. The oven temperature is then slowly dropped. In this process, as the solvent evaporates, it begins to flow in the direction of lower oven temperatures. As this reaction occurs, a metallic crystal is left behind having a highly aligned crystaline structure which results in the material exhibiting superconductive properties at a temperature of approximately 30.degree. K. higher than is achieved when a similar previously used process is used wherein the solvent is evaporated in an oven not having a temperature gradient. It is currently not entirely understood exactly why such a process involving a temperature gradient results in a higher temperature superconductor.
Richardson et al. in the Journal of the American Ceramic Society disclose a method for producing a textured film of superconducting YBa.sub.2 Cu.sub.3 O.sub.7-x, comprising the steps of: mixing Y.sub.2 Cu.sub.2 O.sub.5 and BaCuO.sub.2 so as to produce a mixture Of Y.sub.2 Cu.sub.2 O.sub.5 and BaCuO.sub.2 in a 1:4 mole ratio; ball-milling the mixture in acetone or isopropyl alcohol until producing a precursor powder with an average particle size of about 0.5 .mu.m; mixing the precursor powder with isamyl alcohol so as to form a paste; applying the paste by the doctor blade method to a 1-mm-thick alumina plate so as to form a supported film 25 to 250 .mu.m thick; drying the supported film at about 100.degree. C.; and passing the supported film through a resistively heated steep temperature gradient furnace. In the Journal of Materials Science Letters Richardson et al. disclose such a method which comprises the following steps: ball-milling single-phase YBCO in an equal volume of potassium chloride so as to produce a precursor powder with an average particle size of about 1.0 .mu.m; mixing said precursor powder with iso-amyl alcohol so as to form a paste; applying said paste by the doctor blade method to a 30 mm.times.3 mm.times.1 mm thick sintered MgO plate so as to form a supported film 200 .mu.m thick; drying said supported film at about 100.degree. C.; and passing said supported film lengthwise through a resistively heated steep temperature gradient furnace.
Richardson et al. in the Journal of Materials Science Letters note that bulk conductors capable of carrying currents in excess of 10.sup.5 Acm.sup.-2 at 77 K. are desirable for large-scale practical applications of high-temperature ceramic superconductors. Highly textured material is required for the efficient transport of intergranular currents, due to the strongly anisotropic nature of the superconducting characteristics of YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO). Although well-oriented thin films have been prepared which exhibit critical current-field behavior akin to that of a good single crystal, their uses are limited to electronic device applications, due to the required vacuum equipment and low depositon rates. The logical approach is therefore to develop texture in thick films after they have been applied to the substrate. Previous approaches to melt-texturing produced highly aligned but inhomogeneous regions, because unwanted non-superconducting phases precipitate at the high temperatures required to melt YBCO; hence, the accumulation of secondary phases in the melt limited the length of samples textured by conventional zone melting.
Richardson et al. disclose an improved melt-texturing technique, viz., a "modified travelling solvent zone" (TSZ) technique, which produces highly oriented thick films of YBCO on a ceramic substrate. An appropriate solvent is used for reducing the melting point of the superconductor-solvent mixture to the regime of thermal stability for YBCO. The crystalline superconductor solute precipitates with grain alignment controlled by the preferred growth direction and the solvent concentration gradient. The precast YBCO-solvent mixture is passed through a steep temperature gradient furnace, dissolution and precipitation thereby taking place in a travelling solvent zone; a continuous, homogeneous, YBCO superconductor film is produced.
Recent superconducting material fabrication techniques implementing a melt-texturing process similar to that disclosed by Richardson et al. have utilized temperature gradients in the crystal growth phase at 800.degree. C. temperature ranges or higher in order to facilitate proper crystal growth. These techniques have produced 125.degree. K. rare earth metal oxide superconductors which are stable at approximately 30.degree. K. higher temperatures than conventional 1-2-3 compounds. These newer rare earth materials are of greater utility than the conventional materials because of expected increased stability at liquid nitrogen temperatures.
What is not believed herein to have been previously disclosed in the art is a further improved approach to fabricating textured pseudo-single crystal superconductive materials, one which combines an additional texturing technique or additional texturing techniques with, and thereby enhances the superconductor-producing qualities of, Richardson et al.'s TSZ melt-texturing technique using a steep temperature gradient furnace.